A new study has suggested that seeds of Alzheimer's disease can potentially attach to surgical instruments and be transferred from one person to another during certain medical procedures.
The findings provide the strongest evidence yet of possible dementia transmission in humans via microscopic protein fragments.
Health officials and experts were quick to reassure the public after the highly controversial research was reported in the leading journal Nature.
However the findings prompted speculation about the safety of some medical procedures, including dental treatments.
Blood donations are not considered a meaningful risk, but should be investigated as a precaution, say the researchers.
British scientists stumbled on the discovery while investigating a rare form of "iatrogenic" Creutzfeldt Jakob Disease (iCJD), a brain-destroying condition known to be spread by contaminated surgical instruments and procedures.
They inspected the brains of eight patients who died from the disease after receiving pituitary growth hormone extracted from cadavers.
Unexpectedly, six bore a clear molecular hallmark of Alzheimer's - sticky clumps of fragmented protein called amyloid beta that form among neurons and on the walls of blood vessels.
In four cases, the amyloid deposits were widespread and only one patient was not affected at all.
All eight individuals were relatively young, aged 36 to 51, and none had genetic variants associated with early-onset Alzheimer's disease.
The evidence points to the hormone carrying "seeds" of the Alzheimer's protein into the patients' brains as well as CJD.
Since the prion proteins responsible for iCJD can be transmitted in other ways - for instance, by neurosurgery - experts are not ruling out the possibility that the same is true for the Alzheimer's molecule.
Lead scientist Professor John Collinge, director of the Medical Research Council Prion Unit at University College London, said there was increasing evidence that neurodegenerative diseases including Alzheimer's might, in rare circumstances, be "acquired".
Previous experiments on laboratory mice and monkeys had already shown that transmission of the Alzheimer's protein is theoretically possible.
When liquefied brain tissue from dead Alzheimer's patients was injected into the central nervous systems of the animals, they developed the brain changes associated with the disease.
The scientists do not believe CJD prions somehow triggered the development of amyloid beta deposits in the brains of the growth hormone patients. Brains of 116 patients with prion diseases who had not received pituitary growth hormone did not have the Alzheimer's hallmark, and the two diseases affect different brain areas.
Other research has shown that amyloid beta can form in the pituitary gland, a pea-sized body at the base of the brain.
None of the patients had developed Alzheimer's symptoms and their brains did not show another key change associated with the disease, twisted strands of protein within nerve cells called tau tangles. Whether or not they would have started to suffer symptoms had they lived longer remains an unanswered question.
Questioned specifically about dentistry, Prof Collinge said: "The seeds will potentially stick to metal surfaces whatever the instrument is. With prions, we know quite a lot about that. Certainly, there are potential risks with dentistry where it's impacting on nervous tissue, for example root canal treatments.
"If you are speculating that amyloid beta seeds might be transferred by instruments, one would have to consider whether certain types of dental procedure might be relevant."
He stressed there was no epidemiological evidence at all to suggest that Alzheimer's could be transmitted via blood transfusions, but added: "I think it's not unreasonable to have a look. My concerns would be more to see if there is a risk of seeding from metal surfaces. I think that is something we ought to prioritise."
Later he appeared to backtrack on dental treatments, issuing a statement saying the current data had "no bearing" on dental surgery and "certainly do not argue that dentistry poses a risk of Alzheimer's disease".
Meanwhile, he urged people not to be concerned about planned medical procedures, and to dismiss any notion of Alzheimer's being "contagious" in the same way as flu, for instance.
"No way is this suggesting that Alzheimer's is a contagious disease," he said. "You can't catch it by living with someone who has Alzheimer's disease or being a carer.
"I don't want to cause any alarm. No-one should consider cancelling or delaying any kind of surgery. But I think it would be prudent to do some research in this area going forward."
Tina Leonard, Head of Advocacy and Public Affairs at The Alzheimer's Society of Ireland, said: "While the findings certainly sound alarming, there is currently no evidence that Alzheimer's is a contagious condition that can be transmitted from person to person via a medical procedure.
"We would urge that caution be exercised when interpreting these results.
"What we do know is the biggest risk factor for Alzheimer's is age, along with genetics and lifestyle factors such as smoking, obesity and high blood pressure.
"However, all research that sheds light on potential causes of Alzheimer's and the related dementias is welcome. What we really need is for the concept of "brain health" to inform government policies geared at promoting a healthy lifestyle, which it is believed could prove significant in reducing the risk of dementia."
